the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Sep 2026
Influence of convective organization on the triple oxygen isotope composition of West African monsoon rainfall: insights from a 7-year record in Benin
Abstract. Triple oxygen isotopes, and specifically 17O-excess, provide new opportunities to investigate kinetic fractionation associated with atmospheric moisture sources, continental moisture recycling, and cloud microphysical processes. However, observations of 17O-excess in precipitation remain extremely scarce in Africa, leaving its potential for tracing atmospheric processes in the West African Monsoon (WAM) largely unexplored. Here we analyze a high-frequency precipitation isotope dataset (δ18O, δ2H, d-excess, and 17O-excess) from Benin, comprising a 7-year record at the inland Nalohou station and a complementary 2-year record from a coastal station near Cotonou. Three isotopic clusters are identified inland, corresponding to distinct rainfall regimes. The first is associated with short-lived convective events affected by sub-cloud rain evaporation, while the second reflects more organized convective systems and a weak signature of continental moisture recycling. The third cluster, observed only inland, is characterized by strongly depleted δ18O together with simultaneous decreases in d-excess and 17O-excess, and occurs mainly during the mature monsoon stage. Its rainfall characteristics are consistent with larger and more organized convective systems, with a greater contribution from their stratiform component. At the interannual scale, δ18O variability is more closely related to rainfall characteristics during the mature monsoon phase than to rainfall amount. Overall, these results indicate that, during the WAM mature monsoon phase, precipitation isotopes primarily reflect convective organization and associated cloud microphysical processes, while retaining a weak signature of continental moisture recycling during inland transport. These combined δ18O, d-excess, and 17O-excess data provide new observational constraints for isotope-enabled atmospheric and hydrological models.
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RC1: 'Comment on egusphere-2026-5233', Anonymous Referee #1, 06 Oct 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-5233/egusphere-2026-5233-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-5233-RC1 -
RC2: 'Comment on egusphere-2026-5233', Anonymous Referee #2, 26 Oct 2026
Influence of convective organization on the triple oxygen isotope composition of West African monsoon rainfall: insights from a 7-year record in Benin
General comments
This manuscript presents a valuable seven-year record of precipitation isotopes in Benin, including 17O-excess, and combines these observations with detailed meteorological and statistical analyses of West African monsoon rainfall. Given the scarcity of such isotope datasets in this region, the record represents an important contribution. However, the physical basis linking the isotope variations to the proposed atmospheric processes is less well established than the statistical characterization. It remains difficult to distinguish observed associations from evidence for the proposed causal mechanisms. Therefore, major revision is recommended, as detailed below, to strengthen the physical reasoning and clarify how the observations support the interpretations.
Major comments
(1) Quantitative basis for the isotope interpretation
The meteorological analyses are detailed and quantitative, whereas the discussion of isotope fractionation in Section 4.1 is largely qualitative and based on mechanisms described in cited studies. The manuscript discusses plausible explanations but does not quantitatively assess how well they account for the observed isotope differences. This makes the isotope interpretation less convincing than the preceding statistical analyses.
I am not suggesting that a comprehensive new isotope-enabled convection model is required. However, I am not yet fully convinced that the present discussion sufficiently supports the title's "Influence of convective organization" and the conclusion that isotope variations are "strongly driven" by it. Please consider whether these statements could be expressed more cautiously to reflect the largely qualitative basis of the interpretation.
(2) Relative humidity in the moisture source region
Lines 579-583 argue that oceanic source conditions are unlikely to be a major driver of isotope variations because the stations share similar source regions. However, a common source region does not imply constant evaporation conditions. Seasonal changes in source relative humidity can change both d-excess and 17O-excess even if the source region remains the same.
Please calculate relative humidity with respect to sea surface temperature for the inferred oceanic source region and match it to the isotope sampling periods. Show its relationships with d-excess and 17O-excess, including regression slopes, correlation coefficients, and p-values. If there are no correlations, it will support "influence of convective organization" rather than source humidity.
(3) Accuracy of the isotope literature review
At lines 93-95 and 99-103, three and TWELVE (!) references, respectively, are grouped without identifying which study supports each claim. For example, regarding the 17O-excess, cite studies according to their contributions: the definition and experiments (Barkan and Luz, 2005; Barkan and Luz, 2007); observational verification of humidity dependence (Uemura et al., 2010); rain re-evaporation in African monsoon precipitation (Landais et al., 2010); and model evaluations of convection and re-evaporation (Risi et al., 2013) (NOTE that this is just an quick example how to cite papers. Please review by yourself for correct citations). Each citation should accompany the specific findings.
The statement that paleoclimate interpretations "often rely" on the amount effect (lines 79-86 and 643-644) is also too general. I believe paleocommunity studies have explicitly used isotope-enabled simulations for interpretation for more than 20 years. This means paleostudies do not rely on the apparent "amount effect"; rather, they focus on more physical mechanisms of isotope fractionation. For example, recent African studies use isotope-enabled climate models to examine the controls on isotope variations, including Boateng et al. (2024, doi: 10.1029/2024JD040748) and Shi et al. (2025, doi: 10.1029/2024PA004999).
(4) Time series of the measured isotope data
The isotope measurements in Section 3.2.1 (lines 303-313) are a major result of this study. However, Figure 3 combines different years. Please first show the measurements as a time series, with calendar date on the horizontal axis. Include the full inland and available coastal records, retain individual measurements, and show sampling gaps and analytical uncertainties.
(5) Isotope relationships with relative humidity in C1
Lines 504-507 use the C1 isotope relationships with relative humidity (RH) to support sub-cloud evaporation. Correlation coefficients and p-values appear at lines 411-413, but scatterplots and regression slopes are missing. Please plot δ18O, d-excess, and 17O-excess against RH. Show individual observations and fitted regression lines, and report slopes, Pearson r, p-values, and sample sizes.
Specific comments
(1) Line 172, Eq. (1). Please cite the original study for the definition of 17O-excess.
(2) Lines 174-179. Please clarify whether 5 per meg denotes SD or SE. Distinguish replicate SD from the SE (SD of the mean =σ / square root (n)). Commonly, 17O-excess error is reported as SE, not SD.
(3) Line 274. Why is this called a "dual approach" when three tasks are listed?
(4) Figures 5, 6, and 9 are too small. Please enlarge the panels and labels and improve the layout.
(5) Figure 5. Please separate the inland and coastal results into six panels (two stations × three isotope pairs), as the overlapping data are difficult to distinguish.
(6) I feel it is better to move Figure 7 after Figure 5 and reorder the corresponding text.
(7) Lines 310-311. The coastal mean δ18O (-3.55) and δ2H (- 27.32) values give d-excess = 1.1‰, not 11.07‰. Please check these values.
Citation: https://doi.org/10.5194/egusphere-2026-5233-RC2
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